One type of semiconductor sensor is a gas sensor device. Semiconductor gas sensors are used to detect the presence of a particular gas or gasses in an environment to which the sensor is exposed. A common type of gas sensor is a metal oxide semiconductor (MOS) gas sensor. MOS gas sensors, which are also referred to as “thick-film” gas sensors, typically include a heating element and a gas-sensitive portion located between two electrodes. The heating element is activated to heat the gas-sensitive portion to a temperature that is suitable for detecting a target gas. The gas-sensitive portion is a thick-film that is configured to undergo an electrical change in the presence of the target gas. The electrical change of the gas-sensitive portion is detected by an external circuit that is electrically connected to the gas sensor.
FIGS. 24 and 25 show part of a gas-sensitive portion 10 of a prior art MOS gas sensor. The gas-sensitive portion 10 is typically formed from a polycrystalline material that includes numerous grains 20. The region of contact between the grains 20 is referred to herein as a grain boundary 22. The grain boundaries 22 are target sites to which molecules of the target gas bind through a process referred to as adsorption. When adsorption of the target gas occurs, the gas-sensitive portion 10 undergoes the above-described electrical change that is detected by the external circuit.
Chemisorption is one type of adsorption that may occur at the grain boundaries 22 in the presence of the target gas. To illustrate the effects of chemisorption, FIG. 24 includes a graph showing an electrical potential barrier at the grain boundary 22 in the presence of air containing oxygen molecules. For an electron 30 to move through the grain boundary 22, it requires enough energy to overcome the potential barrier, which defines a reference magnitude measured in electronvolts (eV). A combination of the potential barriers of all/most of the grain boundaries 22 in the gas-sensitive portion 10 contributes to an electrical resistance of the gas-sensitive portion.
In FIG. 25, the exemplary grain boundary 22 is shown in the presence of molecules of a reducing gas. Chemisorption of the reducing gas has caused a reduction in the magnitude of the potential barrier due to donor electrons from the reducing gas. When the potential barriers are combined, the overall electrical resistance of the gas-sensitive portion 10 is reduced due to the reduction in the magnitude of at least some of the potential barriers at the grain boundaries 22 at which reduction has occurred. The exemplary reduction in electrical resistance of the gas-sensitive portion 10 is detectable by the external circuit connected to the gas sensor as being indicative of the presence of a target gas. Although not shown, in the presence of an oxidizing gas, the magnitude of the potential barrier increases, thereby resulting in an increase in the electrical resistance of the gas-sensitive portion 10, which is also detectable by the external circuit connected to the gas sensor as being indicative of the presence of a target gas.
Heterogeneous catalysis is another process that may occur at the grain boundaries 22, depending on the type gas near the gas-sensitive portion 10. One example of heterogeneous catalysis, referred to as carbon monoxide (CO) oxidation, results in the oxidation of a carbon dioxide (CO2) molecule, due to the presence of a carbon monoxide molecule and an oxygen molecule located near one of the grain boundaries 22 of the gas-sensitive portion 10. Heterogeneous catalysis, in at least some instances, results in an electrical change of the gas-sensitive portion 10, which is detectable by the external circuit connected to the gas sensor as being indicative of the presence of a target gas.
In addition to preparing the gas-sensitive portion for detecting and/or exposure to the target gas through adsorption, the heating element is also activated to “reset” the gas sensor through a process referred to as desorption. During desorption molecules are evacuated from the gas-sensitive portion in order to prepare the sensor for sensing additional quantities of the target gas or for sensing a different type/species of target gas.
When the heating element of the typical MOS gas sensor is activated, other portions of the gas sensor are heated in addition to the gas-sensitive portion. For example, if an intermediary layer is located between the heating element and the gas-sensitive portion, then the heating element heats the intermediary layer in addition to heating the gas-sensitive portion. Furthermore, if the heating element is positioned in contact with or in proximity to a base layer, a substrate layer, or a handle layer, then heat energy from the heating element may undesirably/unnecessarily be transferred thereto. Additionally, since the gas-sensitive portion of a MOS gas sensor is a “thick-film,” heating of the gas-sensitive portion has an associated time-constant that may be of longer duration than desired. Accordingly, in the typical MOS gas sensor, energy consumed by the heating element is used to heat portions of the gas sensor that are not desired to be heated, and heating the gas-sensitive portion may consume more time than desired.
Therefore, for at least some of the above-described reasons, it is desirable to structure the gas sensor so that the heat energy generated by the heating element heats the gas-sensitive portion of the gas sensor quickly and without significantly heating other parts of the gas sensor. Accordingly, further developments in the area of gas sensors are desirable.